1. Field of the Invention
The present invention relates to an improvement in the method of forming an integral hollow rivet in deformable sheet material wherein the rivet is used to attach a tab thereto. More particularly, this invention pertains to an improvement in forming an integrally riveted pull tab on a can end whereby the panel remains rigid because excess metal is not pushed into the central panel, and, therefore, no portion of the panel tends to protrude upwardly of the can end.
2. Description of the Prior Art
The first operation of forming an integral rivet for an easy open can end or the like consists of forming an outwardly projecting bubble in the central portion of a can end. When reforming this bubble into a hollow rivet or button in a second operation, a portion of the sheet metal in the bubble tends to flow into the uncompressed area of the central portion of the can end. Any disposition of additional sheet material into the uncompressed panel area causes the central portion to be more susceptible to bulging and other surface deformities in response to internal and external pressures against the can end. Although bulging may occur regardless of the thickness of the metal in the can end, such end panel deformities are particularly acute when thin gauge metal is used for the can end, such as aluminum alloy 5352 of H19 temper having a gauge of about 0.012 inch or less. Localized bulging of the central portion around the formed rivet may cause the tab attached thereto to protrude upwardly of the general plane of the central portion on occasion. It is known that an upwardly protruded tap displaced in response to bulging of the central portion often retains such displaced configuration even after the bulge has receded.
Bulges in a can end and upwardly protruding pull tabs on easy opening can ends have numerous disadvantages. First, the protruding tab is more susceptible to premature opening of the can, which may easily result during handling and transportation of such cans. Also, an upwardly protruding bulge in the central portion may be excessive enough to cause can stacking difficulties. Finally, bulges in the can end or displaced tabs present an unsightly appearance detracting from the desired commercial impression of a container.
It has been taught in the prior art that the area of the central portion about the hollow rivet or button may be coined. Such coining operation causes the excess metal to flow outwardly of the button into a localized area of the central portion where bulging may occur. Attempts to prevent such outward metal flow include clamping the area of the central portion between a set of dies, but such compression adds a cost to the forming operation and does not insure that the metal does not flow beyond the compression dies and may, therefore, still result in bulging of that area of the central portion.
The prior art also teaches the simultaneous formation of an annular groove at the base of the outwardly projecting bubble as the bubble itself is formed in the first operation. Such annular groove, which is formed in metal that lies outside the dimension of the bubble, facilitates the lifting of the pull tab. Such groove does not prevent bulging in the area of the central portion outside the finally formed rivet.
Accordingly, an improved method of forming an integral hollow rivet in deformable sheet material is desired to prevent unwanted surface deformities in the sheet material about the rivet.